My DD played with these pencils and she didn't wash the fabric first, and used the fabric medium, which made it quite stiff. We did wash it (by hand) after the fabric medium dried which made it a bit softer but it is still quite stiff.
When I get a chance I want to play with them. I would try washed and unwashed fabric then draw on them, I want to try dry pencils and also dipping them in water. I would do little samples of every different combination you could think of and mark it on the fabric with permanent marker. Then you have your own little reference.